Refactor jtx::Env:

These changes eliminate the Env's OpenLedger member and make
transactions go through the Application associated with each
instance of the Env, making the unit tests follow a code path
closer to the production code path.

* Add Env::open() for open ledger
* Add Env::now()
* Rename to Env::current()

* Inject ManualTimeKeeper in Env Application
* Make Config mutable
* Move setupConfigForUnitTests
* Launch Env Application thread
* Use Application ledgers in Env
* Adjust Application clock on ledger close
* Adjust close time for close resolution
* Scrub obsolete clock types
* Enable features via Env ctor
* Make Env::master Account object global

* Cache SSL context (performance)
* Cache master wallet keys in Ledger ctor (performance)

src/ripple/app/tests/Offer.test.cpp

@@ -149,8 +149,11 @@ public:
 
         auto const switchoverTime = STAmountCalcSwitchovers::enableUnderflowFixCloseTime ();
 
-        for (auto timeDelta : {-10s, 10s}){
-            NetClock::time_point const closeTime = switchoverTime + timeDelta;
+        for (auto timeDelta : {
+            - env.closed()->info().closeTimeResolution,
+                env.closed()->info().closeTimeResolution} )
+        {
+            auto const closeTime = switchoverTime + timeDelta;
             STAmountCalcSwitchovers switchover (closeTime);
             env.close (closeTime);
             // Will fail without the underflow fix